RADIOENGINEERING, VOL. 20, NO. 4, DECEMBER 2011 785

Millimeter-Waves Structures on Benzocyclobutene Dielectric Substrate

Sandra COSTANZO 1, Antonio BORGIA 1, Ignazio VENNERI 2, Giuseppe DI MASSA 1

1 Dipartimento di Elettronica, Informatica e Sistemistica, University of Calabria, 87036 Rende (CS), Italy 2 Saipem S.P.A. Rome Project Execution Centre, Dep. of Automation and Instrumentation, Vibo Valentia, Italy

costanzo@deis.unical.it

Abstract. The need of low-loss substrate materials with stable dielectric performances is a strong requirement when working at millimeter frequencies, where standard dielectrics exhibit prohibitive losses. In this paper, the authors focus their attention on a polymer material, the benzocyclobutene (BCB), having a low dielectric constant and a low loss tangent, with a stable behavior up to THz frequencies. A specific in-house manufacture technology is described to realize millimeter-wave structures on a BCB dielectric substrate. Experimental validations on BCB-based circuits and antennas prototypes are discussed.

Keywords Millimeter-wave, benzocyclobutene, low-loss mate-rials.

1. Introduction In recent years, strong research efforts have been

devoted to the investigation of innovative materials and/or fabrication technologies for the realization of efficient low-loss circuits and antennas well working at millimeter frequencies but requiring a minimal increase in cost. To guarantee good performances as dielectric substrate at millimeter waves, the material should have low losses and a low dielectric constant, stable within the operating frequency range, so the values of material permittivity and dissipation factor should be accurately considered, but also the thermal stability of these parameters has a relevant role for a proper material selection, especially for space appli-cations. Potential substrates with excellent performances extending throughout the millimeter-wave range can be found in the polymer category. Among different polymer materials, such as PDMS [1], Polymide [2], Parylene-N [3], whose electrical parameters are summarized and discussed in [4], our attention has been focused on BCB as giving low values of permittivity and loss tangent, together with a low coefficient of thermal expansion, thus guaran-teeing a stronger dielectric stability versus temperature [4]. BCB has already been successfully applied in literature as a covering film for packaging and interconnections on

a silicon substrate [5-9]. In this paper, the use of BCB as substrate material for planar microstrip structures is dis-cussed in order to overcome difficulties related to the occurrence of large dielectric losses in the high microwave range. Furthermore, a low cost in-house fabrication tech-nology is described to realize single-layer and multilayer substrates from a small quantity of BCB liquid material.

2. BCB Dielectric Properties BCB is a promising organic material showing stable

permittivity values and low losses over a broad frequency range. The producer [10] claims a dielectric constant r = 2.65, with a few percent variations between 10 GHz and 1.5 THz, and a loss tangent between 0.0008 and 0.002 from 1 MHz to 10 GHz. Additional data are also available [10] in the frequency range between 400 GHz and 1500 GHz, which confirm a stable dielectric behavior of the BCB on a broad frequency range. However, no specific electrical values are provided in the middle microwave range, below 400 GHz. In order to guarantee the accurate design and performance level of BCB-based microstrip structures also in the uncovered frequency range, a broad-band dielectric characterization has been performed by the authors up to 65 GHz [11]. A conductor-backed coplanar waveguide (CBCPW) has been adopted as test structure and the dielectric parameters of BCB have been extracted from on-wafer S-parameter measurements. As reported in Fig. 1, an approximately steady value near the manufac-turer specification r= 2.65 is obtained, and a close agree-ment with the simulation results can be observed within the measurement frequency range [11]. The extracted values of the loss tangent, reported in Fig. 2, show a variation be-tween 0.001 and 0.009 in the measurement range between 11 GHz and 65 GHz [11].

3. Manufacturing Process of BCB-Based Microstrip Structures The fabrication process for BCB-based microstrip

structures is fully performed into the Microwave Labora-tory at University of Calabria. A three steps procedure,

786 S. COSTANZO, A. BORGIA, I. VENNERI, G. DI MASSA, MILLIMETER-WAVES STRUCTURES ON BENZOCYCLOBUTENE …

essentially based on deposition, curing and etching, is adopted to realize single-layer or multi-layer structures, as described in the process flow-chart of Fig. 3. The BCB is first deposited on the copper ground plane, by using a spin coater at a speed of 1000 rpm, which gives a dielectric thickness equal to 26 m. A soft-cure process is then ap-plied in a convection oven under nitrogen at a temperature of 210°, to avoid polymer oxidation. The two processes of deposition and soft-curing are then repeated for all subse-quent dielectric layers. In order to achieve a full 100% polymerization, a hard-cure process is applied in a convec-tion oven under nitrogen at a temperature of 250°. The top copper layer is then deposited by a physical vapor deposi-tion and the circuit pattern is finally etched by using a laser and photo-etching procedure.

Fig. 1. Dielectric constant of BCB substrate [11].

Fig. 2. Loss tangent of BCB substrate [11].

4. CBCPW on BCB Substrate A first validation test of the manufacturing process

for BCB-based microstrip structures has been performed on a CBCPW configuration. As a matter of fact, the copla-nar structure is widely adopted at high frequencies in alter-native to microstrip lines, because providing best features in terms of dispersion and radiation losses, and also for its easy fabrication and integration capability. To simplify the realization process, the value of BCB thickness is chosen

equal to the maximum height for single coating (26 m for the adopted Cyclotene series 3022 [10]).

Fig. 3. Process sequence and conditions for BCB-based

microstrip structures.

In this case, a hard-cure process is adopted which is typically carried out for realizing a single polymer layer, and leads to achieve 100% conversion from liquid to solid. The central conductor width W and the ground strip sepa-ration G are chosen by simulations on commercial Ansys software to match a 50 characteristic impedance. The values of loss tangent previously determined (Fig. 2) are used for the accurate characterization in the simulation stage. A photograph of the realized 7 mm length CBCPW on BCB substrate is reported under Fig. 4. The experi-mental validation of the CBCPW prototype is performed by using a vector network analyzer Anritsu 37397B and a probe station fitted with 500 m GGB GSG contact probes (Fig. 5).

Fig. 4. Photograph of the realized CBCPW on BCB substrate

(W = 70 m, G = 30 m ).

The comparison between measured and simulated CBCPW insertion loss is illustrated in Fig. 6. The non-perfect agreement between them can probably be attributed to some non-calibrated measurement inaccuracies, primar-ily given by the probe contact and the positioning errors.

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Fig. 5. Photograph of the test setup for the experimental

validation of CBCPW on BCB substrate.

Fig. 6. Comparison between simulated and measured insertion

loss of CBCPW on BCB substrate.

5. V-band Patch Antenna on BCB Substrate As a further validation example, a V-band inset patch

antenna has been designed on a BCB dielectric substrate. The layout of the antenna prototype, with the full indica-tion of all dimensions, is reported in Fig. 7. In order to perform on-wafer measurements, a microstrip-to-coplanar waveguide transition is also included in the design, as illustrated in Fig. 7. The patch antenna is realized on a single layer of BCB, having thickness equal to 26 m. The simulation characterization is performed by assuming the exact loss tangent at the design frequency as reported in Fig. 2, approximately equal to 0.009. In the realization stage, a 0.5 mm copper layer is first adopted as deposition support for the BCB dielectric layer. A hard-cure process is then applied to realize the BCB polymerization, and a 1m copper layer is subsequently deposited by the vaporization procedure.

The antenna layout is finally etched by a laserwriter machine. A photograph of the realized V-band antenna prototype, with a particular showing the microstrip-to-coplanar waveguide transition, is reported in Fig. 8. The test setup adopted to perform on-wafer measurements is illustrated in Fig. 9, and the excellent comparison between the simulated and the measured return loss is reported in Fig. 10.

Fig. 7. Layout and dimensions of V-band patch antenna on

BCB substrate.

(a)

(b)

Fig. 8. (a) Photograph of the realized V-band patch antenna and (b) particular of the microstrip-to-coplanar wave-guide.

6. Conclusion The use of BCB polymer as dielectric substrate for

millimeter-wave circuits and antennas has been discussed in this paper. A specific in-house manufacture technology has been developed to realize at low cost BCB-based mi-crostrip structures of arbitrary thickness by polymerization of small quantities of liquid BCB. The effectiveness of the

57 m

29 m

788 S. COSTANZO, A. BORGIA, I. VENNERI, G. DI MASSA, MILLIMETER-WAVES STRUCTURES ON BENZOCYCLOBUTENE …

fabrication methodology has been successfully tested on millimeter-wave prototypes of coplanar waveguides and patch antennas.

Fig. 9. Photograph of the test setup for the experimental characterization of V-band patch antenna on BCB substrate.

Fig. 10. Comparison between simulated and measured return

loss of V-band patch antenna on BCB substrate.

References

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[2] HD MicroSystems. PI-2525, PI-2555, PI-2556 & PI-2574 Polymide. [Online]. Available at: http://www2.dupont.com.

[3] Specialty Coating Systems. SCS Parylene Properties. [Online]. Available at: http://www.scscoatings.com.

[4] COSTANZO, S., VENNERI, F., BORGIA, A., VENNERI, I., DI MASSA, G. 60 GHz microstrip reflectarray on a benzocyclobutene dielectric substrate. IET Science, Meas. Technology, 2011, vol. 5, no. 4, p. 134 - 139.

[5] CARRILLO-RAMIREZ, R., JACKSON, R. W. A highly inte-grated millimeter-wave active antenna array using BCB and silicon substrate. IEEE Trans. Microwave Theory and Tech., 2004, vol. 52, no. 6, p. 1648 - 1653.

[6] SEOK, S., ROLLAND, N., ROLLAND, P. Design, fabrication, and measurement of benzocyclobutene polymer zeo-level packag-ing for millimetre-wave applications. IEEE Trans. Microwave Theory and Tech., 2007, vol. 55, no. 5, p. 1040 - 1045.

[7] GRZYB, J., RUIZ, I., COTTET, D., TROSTER, G. An investigation of the material and process parameters for thin-film MCM-D and MCM-L technologies up to 100 GHz. In Proceedings of the 2003 Electronic Components and Technology Conf., 2003, p. 478 - 486.

[8] KRISHNAMURTHY, V. B., COLE, H. S., SITNIK-NIETERS, T. Use of BCB in gih-frequency MCM interconnects. IEEE Trans. Comp., Packag., and Manufact. Tech. – Part B, 1996, vol. 19, no. 1, p. 42 - 47.

[9] CHINOY, P. B., TAIADOD, J. Processing and microwave characterization of multilevel interconnects using benzocyclobutene dielectric. IEEE Trans. Comp., Packag., and Manufact. Tech. – Part B, 1993, vol. 16, no. 7, p. 714 - 719.

[10] Dow Chemical. Cyclotene 3000 Series Advanced Electron Resins. [Online]. Available at: http://www.dow.com/cyclotene.

[11] COSTANZO, S., VENNERI, I., DI MASSA, G., BORGIA, A. Benzocycloutene as substrate material for planar millimeter-wave structures: dielectric characterization and application. Journal of Infrared, Millimeter and Terahertz Waves, 2010, vol. 31, no. 1, p. 66 - 77.

About Authors ... Sandra COSTANZO was born in Cosenza, Italy. She received the Laurea degree (summa cum laude) in Com-puter Engineering from the University of Calabria and the Ph.D. degree in Electronic Engineering from the University of Reggio Calabria in 1996 and 2000, respectively. Since 1996 she has been with the Research Group in Applied Electromagnetics at the University of Calabria, where cur-rently she is an Assistant Professor with the Faculty of Engineering. Since 1996 she has been involved in many research projects funded by ESA, ASI, MIUR and private companies. She is Senior member of IEEE, member of IEEE South Italy Geoscience and Remote Sensing Chapter, CNIT (Consorzio Nazionale Interuniversitario per le Tele-comunicazioni) and SIEm (Società Italiana di Elettromag-netismo). Her research interests are focused on near-field far-field techniques, antenna measurement techniques, antenna analysis and synthesis, numerical methods in elec-tromagnetics, millimeter-wave antennas, reflectarrays, synthesis methods. She has (co)authored more than 90 contributions in international journals and conferences.

Antonio BORGIA was born in Reggio Calabria (Italy). He received the Laurea degree in Computer Engineering from the University of Calabria in 2008 and currently he is a PhD Student in System and Computer Engineering at the University of Calabria. He is Student member of IEEE, member of CNIT (Consorzio Nazionale Interuniversitario per le Telecomunicazioni) and SIEm (Società Italiana di

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Elettromagnetismo). His research interests are focused on millimeter-wave antennas and technologies, printed anten-nas, reflectarrays. He has (co)authored more than 15 con-tributions in international journals and conferences.

Ignazio VENNERI was born in Catanzaro, Italy, in 1980. He received the Information Technology Engineering Lau-rea degree from the University of Calabria in 2005 and the Ph.D degree in Electronic Engineering from the University “Mediterranea” of Reggio Calabria in 2009. From January 2009 to November 2010 he was a Contract Researcher in Electromagnetic Waves at the University of Calabria. Since December 2010 he has been employed at Saipem S.P.A. - Rome Project Execution Centre – Vibo Valentia - Auto-mation and Instrumentation Department as Telecommuni-cation Project Specialist.

Giuseppe DI MASSA was born in Barano d'Ischia (Na), Italy, in 1948. He received the Laurea degree as Doctor in Electronic Engineering from the University of Naples in 1973. From 1978 to 1979 he was a professor of Antennas at the University of Naples. In 1980 he joined the Univer-

sity of Calabria as a professor of Electromagnetic Waves. Since 1985 he served as an Associate Professor and since 1994 as a Full Professor at the same university. From 1985 to 1986 he was a Scientific Associate at CERN, Geneva. In 1988 he was a Visiting Professor at Brookhaven National Laboratory in Long Island (NY) USA. From 1997 to 2002 he was the Dean of the Department of Elettronica, Infor-matica and Sistemistica and the President of Programming Committee at the University of Calabria. From 2002 to 2007 he was WP leader in the Network of Excellence ACE 'Antenna Centre of Excellence' of European Commission. At present he is the chairman of the Course in Telecommu-nication Engineering at the University of Calabria, the Italian Delegate in the European COST IC0603 “Antenna Systems & Sensors for Information Society Tecnologies”. His main research interests are focused on applied compu-tational electromagnetic, microstrip antennas, microwave integrated circuits, gaussian beam solutions, millimeter wave antennas, reflectarrays, near-field measurements. He has (co)authored more than 250 contributions in interna-tional journals and conferences.